A shutoff device for a power-assisted steering system is known for instance from German Patent Disclosure DE 42 21 459. Here, in an end position of the working piston, which is at high pressure, the pressurized pressure chamber communicates with the pressure chamber that is subject to return pressure. The hydraulic oil can therefore drop in pressure and flow out via the pressure connection of the inactive pressure side. This relief course in the end positions of the servomotor proceeds via a longitudinal groove in the inner wall of the servo cylinder that comes to communicate with the pressure connection. This communication in the end positions is established by having the working piston overtake the longitudinal groove. On steering back into the other direction, however, such a shutoff device is initially incapable of building up any pressure, since the working piston must first cover a short distance before it can close the longitudinal groove. During this time, no servo force is available, and the driver must bring all the steering force to bear manually.
From French Patent Disclosure FR-A 1 125 999, another control motor with a shutoff device for a working piston is known in which a movable ball and spring unit is inserted into the working piston. A ball seals off a bypass channel around whichever pressure chamber is relieved at the time at a sealing edge of the working piston. By contact with a protruding edge in the two end positions of the servo cylinder, a respective ball is forced away from the associated sealing edge, so that the two pressure chambers enter into communication. With this arrangement, however, only a bypass channel with a very small flow cross section can be opened. Another control motor with a shutoff device is known from U.S. Pat. No. 4,860,646, As disclosed a small control piston protruding to both sides is disposed in the working piston, and by means of the control piston, a relatively narrow bypass channel is opened when the working piston arrives at the end wall of the cylinder.